Carbon brush and method and material for the production thereof

ABSTRACT

Carbon brush comprising a base body produced from carbon material by pressing and optionally sintering and a layer of a metallic material applied to an outer surface of the base body by pressing and possibly sintering, the metal layer being connectable to a metallic carrier by welding or soldering.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of PCT Patent Application No.PCT/EP04/014235 filed on Dec. 14, 2004.

BACKGROUND OF THE INVENTION

Carbon brushes for electric motors or generators must be secured to asupport which ensures the necessary application pressure on thecommutator or the like and permits the follow-up movement correspondingto the wear of the carbon brush. A carrier for the carbon brush which isparticularly suitable because of its simplicity, cheapness and lowweight is a leaf spring which in addition to the function of supportingand guiding can also perform the function of current transmission.However, the problem arises of securing the carbon brush to the leafspring in such a manner that on the one hand the necessary mechanicalstrength in long-term operation and on the other the transmission oflarge currents is ensured.

From DE 102 07 406 A1 a carbon brush is secured to a leaf spring servingas carrier by welding or soldering and for this purpose is tin-plated,nickel-plated or coated with metal in another manner at the surface tobe joined to the carrier.

From DE 40 40 002 A1 a carbon brush is known for a fuel pump motor whichis soldered to a leaf spring serving as carrier and for this purpose hasa solderable layer which consists of a copper layer and a tin layerapplied thereto. However, with such metallizations, as a rule metallayers applied by electroplating, it is difficult to obtain a solder orweld connection between the carbon brush and leaf spring which has amechanical, thermal and electrical load capacity adequate for long-termoperation. In particular, the stress compensation between the carbonbrush and the leaf spring involves problems due to the difference inmodulus of elasticity and thermal coefficient of expansion.

From DE 2444957 A1 it is known to produce a carbon brush provided with asolderable or weldable metallic layer by jointly compressing a layer ofpulverulent carbon-containing material and a layer of metal powder andthen conducting a heat treatment. The metal powder consists of copper.

From U.S. Pat. No. 3,601,645 it is known to produce an electricalcontact brush having a base body consisting of metal and carbon materialand a metal layer undetachably connected to said base body by pressingand sintering a pulverulent metal-carbon material for the base body anda metal powder, wherein the metallic layer can consist of copper, ironor an alloy.

From DE 32 17 217 C2 it is known to produce a contact brush by pressinga bundle of metal-coated carbon fibres, wherein the metal coating of thecarbon fibres can consist inter alia of copper, aluminium, tin, iron oran alloy thereof.

SUMMARY OF THE INVENTION

The invention is based on the problem of providing a carbon brush and amethod and a material for the production thereof which permits a simplebonding to a leaf spring which can bear high mechanical and electricalloads.

BRIEF DESCRIPTION OF THE DRAWING

The FIGURE is a schematic view of a carbon brush welded to a leafspring.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The invention will be explained hereinafter in detail with reference toan example of embodiment.

It is commercially of interest to weld carbon brushes to leaf springsbecause this enables a bond to be obtained through which high currentsof more than 15 Å can be transmitted. This would enable this cheaperdesign to be used also for motor types which because of the high demandsregarding the current strength in the blocking test or also incontinuous operation hitherto could not be produced using this concept.Motors of this type, in which the carbon brush with small dimensionsmust transmit high currents, are for example actuators in motorvehicles, for example drive mechanisms for window lifters, seatadjustments, sunshine roofs, mirror adjustments and the like.

According to the invention a carbon brush is provided which apart fromthe usual one-layer or multi-layer base body of pressed and sinteredcarbon material has at least one outer layer of metallic material whichensures the weldability or solderability of the carbon brush, thismetallic layer likewise being made by pressing and sintering andconnected to the base body.

For welding or soldering a carbon brush a suitable welding method (forexample ultrasonic welding, laser welding or the like) or solderingmethod must be applied and furthermore to produce the carbon brusheshaving a weldable metallic layer a material must be available which onthe one hand in the powder-metallurgical processing bonds well to thecarbon material of the base body of the carbon brush and on the otherhand can be readily welded or soldered with the method chosen. It hasbeen found that pure copper and its alloys as well as iron or steel arenot suitable because their coefficient of thermal expansion and modulusof elasticity are not compatible with those of the carbon material ofthe base body of the carbon brush in the sintering method. Materialswhich are similar to the carbon material of the base body of the carbonbrush cannot be employed either because due to the graphite componentcontained therein they cannot be welded.

According to the invention it has been found that a metallic material inthe form of a powder mixture of a base metal component A, a ductile andlow-melting metal component B and possibly further additives C meetsboth requirements at the same time, that is the good bonding by pressingand sintering with the functional carbon material of the base body ofthe carbon brush on the one hand and the good weldability orsolderability on the other hand.

According to the invention component A constitutes the major part,preferably more than 80% of the material and preferably comprises copperor a copper alloy with for example Sn, Zn, Ni or Ag. It is preferably inpowder form having a mean diameter D₅₀ of 15 to 25 μm.

Component B is an additional metal which has a higher ductility andlower melting point than the component A. Preferably, the melting pointis 400° C. or less and the breaking elongation 5% or more. The componentB consists preferably of Sn or an alloy of Sn with for example Cu, Ag,Sb. Zn, Ga, In, Bi or Pb. The metal component B may also consist of Inand/or Bi or an alloy with In or Bi. Its proportion in the total powdermixture of the material is preferably 1.5 to 15% by weight. Theadditional metal B is present preferably in a mean grain size of D₅₀ of15 to 35 μm.

In addition, the material may preferably contain one or morenon-metallic organic and/or inorganic additives C in powder form.Possible additives of this type are for example stearates, graphite,talcum, SiC or A1₂O₃. Their mean grain size D₅₀ is preferably not morethan 20 μm.

A preferred example of the composition of the material is (figures inpercent by weight):

-   -   A: 92% Cu (D₅₀ 15-25 μm)    -   B: 7.5% Sn (D₅₀ 15-55 μm)    -   C: 0.5% graphite (D₅₀ ≦20 μm)

The material indicated can be excellently processedpowder-metallurgically by pressing and sintering and bonds excellentlyto the functional layer (base body) of the carbon brush to form a metallayer having excellent welding and soldering properties. It has beenfound that in the pressing and sintering the low-melting and ductilemetal B wets the base metal A but does not diffuse into the latter, oronly to a slight extent.

It is thereby ensured that the predominant or at least an adequate partof the metal B is not alloyed to the metal A after the sintering but ispresent in pure form and thus the desired ductility of the metal layeris achieved. Due to the ductility of the material a stress compensationis achieved between the base body 1 of the carbon brush and the weldablemetal layer 2 on the one hand and the leaf spring welded to the metallayer 2 on the other hand.

In other words, in the sintered metal layer 2 made by pressing andsintering the base metal A and at least the predominant part of theother metal B form two separate metal phases which although beingintimately connected to each other by the pressing and sintering arenot, however, substantially alloyed with each other.

The heat treatment, that is the sintering of the metal layer, is carriedout at a temperature which is lower than the melting point of the basemetal A but higher than the melting point of the further metal B. Thisis thus preferably a liquid sintering in which the further metal B ispresent in the liquid phase but the base metal A in the solidpulverulent phase. If the base metal A is copper and the further metal Btin, the sintering temperature is advantageously in the range from 300to 700° C., preferably in the range from 400 to 500° C.

The non-metallic organic or inorganic additive C is not wetted by themetal B in the sintering process and thereby prevents any undesirablygreat diffusion of the metal B into the base metal A.

The attached drawing illustrates schematically a carbon brush accordingto the invention welded to a leaf spring.

The carbon brush comprises a base body 1 made in the usual manner in oneor more layers from a conventional carbon material for carbon brushes,in particular a graphite mixture containing additives, by pressing andheat treatment (sintering). The excellently weldable or solderable metallayer 2, which is made from the aforementioned pulverulent materialaccording to the invention likewise by pressing and possibly sintering,is undetachably connected or bonded to the base body 1. The base body 1and weldable metal layer 2 may be simultaneously made and bondedtogether by pressing and sintering or the base body 1 may beprefabricated and the weldable metal layer 2 subsequently formed thereonby pressing and sintering.

The easily weldable metal layer 2 enables the entire carbon brush to besecured to a leaf spring 3 by welding, for example by weld spots 4 orfor instance by a continuous weld seam.

The thickness of the sintered metal layer 2 depends on the requirementsof the welding or soldering technique selected for connecting to theleaf spring 3. The thickness of the metal layer 2 preferably lies in therange from 0.2 to 5 mm. For a soldering connection to the leaf spring 2as a rule a thickness of the metal layer 2 of less than 1 mm, inparticular in the range from 0.2 to 0.5 mm, should suffice. For a weldjoint as a rule a thickness of at least 1 mm, for example in the rangefrom 2 to 4 mm, would be advantageous.

1. Carbon brush comprising a base body produced from carbon material anda metal layer which is applied to an outer surface of the base body andcan be connected by welding or soldering to a metal carrier, the metallayer being a layer made from metal powder by pressing and optionallysintering and undetachably bonded to the base body by pressing andoptionally sintering, characterized in that the metal layer contains abase metal (A) and at least one further metal (B) of higher ductilityand lower melting point than the base metal (A), the further metal (B)being present at least predominantly as a metal phase which is separatefrom the base metal (A) and is intimately bonded to the base metal (A)by pressing and optionally sintering but predominantly does not form analloy therewith.
 2. Carbon brush according to claim 1, characterized inthat the further metal (B) is so chosen that at sintering temperature inthe liquid phase it wets the base metal (A) but does not diffuse intosaid base metal, or only slightly.
 3. Carbon brush according to claim 1,characterized in that the further metal (B) is composed of tin, indiumor bismuth or an alloy thereof.
 4. Carbon brush according to claim 1,characterized in that the further metal (B) has a melting point of notmore than 400° C. and a breakage elongation of not less than 5%. 5.Carbon brush according to claim 1, characterized in that the furthermetal (B) is present in a proportion between 1.5 and 15% by weight ofthe metal layer.
 6. Carbon brush according to claim 1, characterized inthat the metal layer additionally contains an additive (C) ofnon-metallic, organic and/or inorganic material.
 7. Carbon brushaccording to claim 6, characterized in that the non-metallic additive(C) contains stearates, graphite, talcum, SiC or Al₂O₃.
 8. Carbon brushaccording to claim 1, characterized in that the metal layer thereof isconnected to a carrier of metal by welding or soldering.
 9. Carbon brushaccording to claim 8, characterized in that the carrier is a leafspring.
 10. Material for producing a weldable and/or solderable layer ofa carbon brush comprising a powder mixture which contains a pulverulentmain metal (A), in particular copper or copper alloy, and a pulverulentadditive metal (B) of greater ductility and lower melting point than themain metal, the main metal (A) being copper or a copper alloy and theadditive metal (B) being tin, indium or bismuth or an alloy thereof. 11.Material according to claim 10, characterized in that it contains apulverulent additive (C) of non-metallic, organic and/or inorganicmaterial.
 12. Material according to claim 10, characterized in that theadditive metal (B) is present in a proportion of 1.5 to 15% by volume ofthe powder mixture.
 13. Method for producing a carbon brush comprising abase body of carbon material and a metal layer undetachably bondedthereto, comprising the steps: a) producing the base body by pressingand sintering of a pulverulent carbon-containing material; b)simultaneously with or following step a): producing the metal layer bypressing and sintering a pulverulent metallic material, the metal layerbeing bonded undetachably to the base body by pressing and sintering; c)the pulverulent metallic material contains a mixture of a pulverulentbase metal A and at least one pulverulent further metal B of greaterductility and lower melting point than the base metal A; and d) thefurther metal B and the sintering temperature are so selected thatduring the sintering the further metal in the liquid phase wets the basemetal A but does not, or only slightly, diffuse into the latter, wherebythe further metal B is present at least predominantly as a metal phaseseparate from the base metal A and predominantly not alloyed therewith.14. Method according to claim 13, characterized in that the main metal Acomprises copper or copper alloy and the further metal B comprises tin,indium or bismuth or an alloy thereof.
 15. Carbon brush according toclaim 2, characterized in that the further metal (B) is composed of tin,indium or bismuth or an alloy thereof.
 16. Carbon brush according toclaim 2, characterized in that the further metal (B) is present in aproportion between 1.5 and 15% by weight of the metal layer.
 17. Carbonbrush according to claim 3, characterized in that the further metal (B)is present in a proportion between 1.5 and 15% by weight of the metallayer.
 18. Carbon brush according to claim 4, characterized in that thefurther metal (B) is present in a proportion between 1.5 and 15% byweight of the metal layer.
 19. Carbon brush according to claim 2,characterized in that the metal layer additionally contains an additive(C) of non-metallic, organic and/or inorganic material.
 20. Materialaccording to claim 11, characterized in that the additive metal (B) ispresent in a proportion of 1.5 to 15% by volume of the powder mixture.